This invention relates to a method of controlling gas density to maintain a high level of ionization in ionized physical vapor deposition (IPVD) apparatus, which are widely used in the fabrication of semiconductor devices.
Physical vapor deposition (PVD or sputtering) is a well-known process for depositing thin solid films on substrates, and is widely practiced in the semiconductor industry. Ionized physical vapor deposition (IPVD), also referred to as ionized metal plasma (IMP) deposition, has been used more recently to deposit metal films (notably copper) in etched structures such as vias and contact holes.
IPVD deposition apparatus differs from earlier versions of PVD apparatus in that a metal film is deposited on the substrate using a flux of ionized metal atoms. The metal atoms are ionized in a dense plasma generated in the process chamber. A typical IPVD apparatus is shown schematically in FIG. 1. The working gas in the process chamber 100 is typically an inert gas such as argon; the gas pressure is approximately 25 mTorr. During a deposition process, the working gas flows into the process chamber and is removed by a vacuum system (not shown). An RF coil (not shown) creates a plasma 103 in the working gas between a magnetron cathode 102 and the substrate 1. Metal atoms 101 are sputtered from the magnetron cathode 102; the atoms are ejected from the cathode with a kinetic energy of about 5 eV, and undergo ionizing collisions in the plasma 103. The result is that a flux of metal ions 104 crosses the gap between the plasma 103 and the substrate 1 (typically a silicon wafer), which has a negative bias with respect to the plasma.
Since the gas pressure is approximately 25 mTorr, the mean free path xcex of the metal atoms is approximately 1 to 2 cm. However, the mean free path for ionization xcexi. is approximately 30 to 40 cm. This means that multiple collisions in the plasma are required to produce a useful degree of ionization. FIG. 2 illustrates a dense plasma 103 in which a metal atom 201 (sputtered from the magnetron cathode 102) undergoes many collisions with both neutral and ionized gas atoms, so that a metal ion 204 is produced.
The kinetic energy of the metal atoms is deposited in the working gas and the plasma. This leads to a problem as the magnetron power level is increased, thereby increasing the flux of metal atoms entering the plasma. Heating of the working gas and plasma by the metal atoms causes the working gas and plasma to rarefy, so that fewer collisions occur; the result is a lower probability of ionization of the metal atoms. This situation is illustrated in FIG. 3. Plasma 303 is hotter and therefore less dense than plasma 103, so that metal atom 201 experiences only a few collisions and traverses the plasma without being ionized.
In general, as the IPVD process is scaled up to higher magnetron power levels, the degree of ionization of the metal atoms decreases and the advantage of the IPVD process (that is, depositing metal films from ions rather than neutrals) also decreases.
The present invention addresses the above-described need by providing a method for maintaining a high gas and plasma density in an IPVD device.
The problem of gas rarefaction resulting from magnetron sputtering in IPVD may be avoided by operating the magnetron in a pulsed fashion, rather than in a steady state, during the deposition process. This permits collisions to occur between metal atoms and the gas while the gas is still cold.
In accordance with the present invention, a method for controlling plasma density in an IPVD device is provided which includes the steps of: powering the magnetron during a first time period to produce a flux of atoms which heat the gas; depowering the magnetron during a second time period; and flowing the gas through the device during the powering step and the depowering step. The flow of gas through the device is characterized by a residence time, and the powering and depowering steps are performed in accordance therewith, so as to prevent rarefaction of the gas by heating thereof due to the flux of atoms.
If the residence time is given as xcfx84, and the first time period and the second time period are substantially equal, the operation of the magnetron may be characterized by a frequency of 1/xcfx84. The second time period may be greater than the first time period.